1. Field of Invention
The present invention relates to the technical filed of a substrate device, such as a TFT array substrate device, and a semiconductor substrate device on which thin film transistors (hereinafter, referred to as TFTs) are formed. More particularly, the present invention relates to the technical filed of a substrate device that is appropriately provided with an electro-optical device, such as a liquid crystal device.
2. Description of Related Art
Substrate devices of this type include, for example, a polysilicon film having a source region, a drain region and a channel region, or a semiconductor film, such as amorphous silicon formed on a substrate, such as a quartz substrate. A thermally-oxidized film or the like made by dry oxidation or wet oxidation is formed on the surface of the semiconductor film, or a gate insulation film is formed thereon from an HTO film, a TEOS film, or a plasma oxidation film. Further, a TFT is constructed on the substrate by forming a gate electrode film on the gate insulation film. The TFT is used as a pixel switching element in a TFT array substrate device by being built in each pixel in the image display region of an electro-optical device, such as a liquid crystal device. Otherwise, the TFT also is used as a part of the drive circuit of the substrate device by being built in a peripheral region in the circumference of the image display region.
The substrate device, in which the TFT is built as described above, has been widely used in various types of electro-optical devices, including a liquid crystal device employing a TFT active matrix drive system, and the like.
However, according to a study performed by the inventors of this application, it has been found that when the above-mentioned substrate device is operated for, for example, about 5000 hours, transistor characteristics are deteriorated. More specifically, even if a TFT built in the substrate device exhibits excellent transistor characteristics at the beginning of manufacture of the substrate device, a threshold voltage in the TFT increases in a lifetime of, for example, the 5000 hours which are required in an ordinary product life, and the TFT does not function as a switching element with an ordinary drive voltage, or an off current (that is, a leak current) increases in the TFT and a prescribed duty ratio cannot be sufficiently handled. When the transistor characteristics are deteriorated as described above, a problem arises in that the life of an electro-optical device, which is finally constructed by including, for example, the substrate device, is reduced because the quality of an image, such as a contrast ratio and brightness in the electro-optical device, is lowered. In particular, it also has been found that when various electro-optical devices, including the substrate device, are used in very humid regions, such as South East Asia, and when strong light beams are projected to the substrate device through a light bulb of a projector, the characteristics of a TFT are liable to be more deteriorated because the TFT is affected by high humidity and high temperature. That is, the substrate device has a problem, which is serious to manufacturers in practical applications, in that the life of the device cannot be definitely determined by the affect of the environment in which the device is used, such as humidity and temperature.
An object of the present invention, which was made in view of the above problems, is to provide a substrate device, which includes a TFT, capable of maintaining excellent transistor characteristics for a long period and which is difficult to be affected by an environment in which it is used, such as humidity and temperature and the life of which can be relatively easily increased. It is also an object of the present invention to provide a method of manufacturing the substrate device, as well as to provide an electro-optical device including the substrate device.
To solve the above problems, a substrate device of the present invention includes a substrate, a semiconductor layer formed on the substrate and having a source region, a channel region, and a drain region, a gate insulation film formed on the semiconductor layer in at least the channel region, and a gate electrode film formed on the gate insulation film. The gate insulation film includes a silicon oxide film. Nitrogen atoms exist in at least one of the silicon oxide film and an interface between the silicon oxide film and the semiconductor layer.
According to the substrate device of the present invention, a TFT is constructed by laminating the semiconductor layer, including the channel region and the like, the gate insulation film, and the gate electrode film on the substrate. Here, nitrogen atoms particularly exist in at least one of the silicon oxide film constituting the gate insulation film and the interface between it and the semiconductor layer. As a result, the time-varying deterioration of transistor characteristics, in particular, the deterioration of the transistor characteristics when a transistor is used in an environment of high humidity, high temperature, and the like, can be reduced as compared with a conventional substrate device in which no nitrogen atom exists. With this arrangement, the life of the substrate device can be increased by the provision of the TFT the performance, which can be stably maintained for a long period of time regardless of an environment in which the TFT is used.
Reasons why the deterioration of the transistor characteristics can be lowered in the substrate device of the present invention will be explained below.
First, according to the study of the inventors of this application, when a conventional substrate device, using a dry oxidation film as a gate insulation film, is continuously operated in an ordinary manner, a phenomenon has been confirmed that the threshold value Vth of a gate voltage for turning on a TFT is dislocated toward an enhancement side regardless of whether the TFT is a p-channel TFT or an n-channel TFT. In a substrate device, which includes a power supply circuit capable of supplying a gate voltage up to a predetermined voltage to the TFT, when the threshold value Vth exceeds the predetermined voltage due to the dislocation, it is impossible to operate the substrate device. The phenomenon of dislocation of the threshold value Vth is conspicuous particularly in the p-channel TFT. First, it is contemplated that the transistor characteristics are deteriorated by that the hydrogen atoms in the interface between a gate insulation film and a semiconductor layer are separated therefrom by the heat generated by a channel resistance when a TFT is in operation (for example, the temperature of the transistor increases to about 400xc2x0 C. in the vicinity of a drain region) and the degree of movement of the nitrogen atoms is lowered in the transistor characteristics. Second, a hot carrier implantation phenomenon (that is, a channel hot electron implantation phenomenon and a drain avalanche hot carrier implantation phenomenon) is contemplated as a cause of the deterioration. Third, it is contemplated that the deterioration is caused by water entering the gate insulation film while the substrate device is being manufactured, or after it has been manufactured, and H2O molecules disperse to the vicinity of the interface between the gate insulation film and the semiconductor layer so as to generate a positive charge.
When the gate insulation film is formed of the silicon oxide film, as in the substrate device of the present invention, if oxygen enters between single crystal silicons, they are expanded and Sixe2x80x94Si bonding and deformed Sixe2x80x94Oxe2x80x94Si bonding are generated and act as hole traps. In contrast, when water (hydrogen atoms) enters the vicinity of the interface between the gate insulation film and the semiconductor layer, Sixe2x80x94H bonding is generated and acts as a positive charge of Si+. Further, when water (oxygen atoms and hydrogen atoms) enters the vicinity of the interface, Sixe2x80x94OH bonding is generated and acts as an electron trap center so as to increase an interface level. As described above, it is contemplated that the generation of the hole traps and the positive charge as well as the increase in the interface level are a main factor of the dislocation of the above-mentioned threshold value Vth.
To address this problem, in the substrate device of the present invention, the generation of the hole trap, the positive charge and negative charge in the interface as well as the increase in the interface level, which are the main factor of the above-mentioned dislocation of the threshold value Vth, are reduced by existing nitrogen atoms in the interface between the gate insulation film and the semiconductor layer and in the vicinity of the interface.
Further, in the substrate device of the present invention, a moisture resistance is enhanced by mixedly existing nitride film bonding by the existence of the nitrogen atoms in the interface between the gate insulation film and the semiconductor layer and in the vicinity of the interface. With this arrangement, the amount of water entering the interface between the gate insulation film and the semiconductor layer and the vicinity thereof can be reduced as well as the generation of the positive charge and the increase in the interface level can be reduced by reducing the generation of the above-mentioned Sixe2x80x94H bonding and Sixe2x80x94OH bonding.
In addition to the above, in the substrate device of the present invention, the existence of the nitrogen atoms in the interface between gate insulation film and the semiconductor layer and the vicinity of the interface permits nitrogen atoms to enter a network of atoms in this region. This arrangement can ease the deformation of the interface and further reinforces a weak bonding portion. In particular, since it is contemplated that the relatively high bonding energy of Sixe2x80x94O is reduced by deforming a bonding state, the Sixe2x80x94Si bonding and the deformed Sixe2x80x94Oxe2x80x94Si bonding, Sixe2x80x94H bonding, and Sixe2x80x94OH bonding can be reduced by the existence of the nitrogen atoms. As a result, the interface level and the trap center of an oxide film can be presented from being formed by a hot carrier.
In one aspect of the substrate device of the present invention, the semiconductor layer is formed of any of low temperature polysilicon, high temperature polysilicon, single crystal silicon, and amorphous silicon.
According to this aspect, any of a low temperature polysilicon TFT, a high temperature polysilicon TFT, a single crystal silicon TFT, and an amorphous silicon TFT is constructed on a substrate. However, the transistor characteristics of this TFT can be effectively prevented from being deteriorated by existing nitrogen atoms in the interface between any of a polysilicon film, a single crystal silicon film, and an amorphous silicon film and a silicon oxide film and in the vicinity of the interface, whereby the life of a substrate device including the polysilicon TFT, the single crystal silicon TFT, or the amorphous silicon TFT can be increased.
In another aspect of the substrate device of the present invention, the above gate insulation film is formed of any of a thermally-oxidized film, a HTO film, a TEOS film, and a plasma-oxidized film.
According to this aspect, a gate insulation film can relatively easily form an insulating film of relatively good quality. Further, the deterioration of the transistor characteristics of the TFT can be effectively prevented by existing the nitrogen atoms in the interface between the semiconductor layer and the silicon oxide film and the vicinity of the interface, whereby the life of the substrate device can be increased.
In another aspect of the substrate device of the present invention, the substrate is formed of a quartz substrate.
According to this aspect, a TFT, in which the transistor characteristics thereof are particularly excellent and the deterioration of the transistor characteristics is lowered, can be constructed on the quartz substrate, whereby a substrate device having a high performance can be realized.
In another aspect of the present invention, the nitrogen atoms exist in both of the silicon oxide film and the interface, and have a peak of concentration in the vicinity of the interface.
According to this aspect, the nitrogen atoms are caused to exist so as to have the peak of concentration in the interface between the semiconductor layer and the gate insulation film, and in the vicinity of the interface where the hole traps and the positive charge are greatly generated and where the interface level is greatly increased. The hole trap, positive charge and interface level are contemplated as the main factor of the above-mentioned dislocation of the threshold value Vth. As a result, the generation of the hole traps and the positive charge and the increase in the interface level can be very effectively reduced by the existence of the nitrogen atoms, which permits the deterioration of the transistor characteristics to be effectively lowered.
In another aspect of the substrate device of the present invention, a plurality of rows of thin film transistors, each formed of the semiconductor layer, the gate insulation film, and the gate electrode film, are disposed on the substrate in an array shape.
According to this aspect, a TFT array substrate device can be constructed which is preferably used in an electro-optical device, for example, a liquid crystal device of a TFT active matrix derive system, and the like.
To solve the above problems, a method of manufacturing the substrate device (including the various aspects thereof) of the present invention includes a step of forming the semiconductor layer on the substrate, a step of forming the gate insulation film on the semiconductor layer, a step of introducing, after the gate insulation film is formed, the nitrogen atoms in the gate insulation film, and a step of forming, after the nitrogen atoms are introduced, the gate electrode film on the gate insulation film.
According to the method of manufacturing the substrate device of the present invention, first, the semiconductor layer is formed on the substrate and the gate insulation film is formed on the semiconductor layer. Thereafter, the nitrogen atoms are introduced in the gate insulation film. Accordingly, the structure, in which the nitrogen atoms exist in at least one of the silicon oxide film constituting the gate insulation film and the interface between the silicon oxide film and the semiconductor layer, can relatively easily be obtained using an existing diffusion technology, implantation technology and plasma technology. Thereafter, a TFT is constructed on the substrate by forming the gate electrode film on the gate insulation film.
In an aspect of the method of manufacturing the substrate device of the present invention, the step of introducing the nitrogen atoms includes a step of executing annealing in a gas atmosphere containing the nitrogen atoms.
According to this aspect, when the gate oxide film is formed using an existing furnace (diffusion furnace) or the like, the step of forming the gate oxide film in the gas atmosphere containing the nitrogen atoms or the step of introducing the nitrogen atoms by executing annealing in the gas atmosphere containing the nitrogen atoms can be relatively simply executed.
In this aspect, the above gas may contain at least one of N2O (dinitrogen oxide) gas, NO (nitrogen oxide) gas, and NH4 (ammonia) gas.
When the substrate device is manufactured as described above, nitrogen atoms can relatively easily be introduced in the interface between the gate insulation film and the semiconductor layer and in the vicinity of the interface at a low cost.
In another aspect of the method of manufacturing the substrate device of the present invention, the step of introducing the nitrogen atoms includes a step of executing a plasma technology, lamp annealing, laser annealing or ion implantation.
According to this aspect, nitrogen atoms can be introduced in the interface between the gate insulation film and the semiconductor layer and in the vicinity of the interface by the existing plasma technology, lamp annealing, laser annealing (excimer laser annealing) or ion implantation.
The electro-optical device of the present invention includes the above-mentioned substrate device (including the various aspects thereof) to solve the above problems.
Since the electro-optical device of the present invention includes the above-mentioned substrate device of the present invention, a substrate device is realized which has a long life and which can maintain a stable performance for a long period of time regardless of an environment in which it is used.
The above operation and other advantages of the present invention will be apparent from the embodiment which will be described below.